Magnetic core structures



Nov. 4, 1969 c. E. BURKHARDT ET 3,477,053

MAGNETIC CORE STRUCTURES 5 Sheets-Sheet 2 Filed Nov. 24, 1967 FBG.3B

FIG.4

Nov. 4, 1969 c, BURKHARDT ET AL 3,477,053

MAGNETIC CORE STRUCTURES 5 Sheets-Sheet 5 Filed Nov. 24, 1967 Nov. 4, 1969 Filed Nov. 24, 1967 C. E. BURKHARDT ET AL MAGNETIC CORE STRUCTURES 5 Sheets-Sheet 4 Nov. 4, 1969 c, BURKHARDT ET AL 3,477,053

MAGNETIC CORE STRUCTURES 5 Sheets-Sheet 5 Filed Nov. 24, 1967 United States Patent O 3,477,053 MAGNETIC CORE STRUCTURES Charles E. Burkhardt, Sharon, and Belvin B. Ellis, Pulaski, Pa., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Nov. 24, 1967, Ser. No. 685,558 Int. Cl. Hlllf 27/24 US. Cl. 336-212 11 Claims ABSTRACT OF THE DISCLOSURE A magnetic core of the stacked type for electrical inductive apparatus which has stepped-lap joints between the leg and yoke portions of the core. The voids produced by the stepped laminations at the intersections of the leg and yoke portions are distributed between certain of the inner and outer corners of the core, and the voids at the inner corners are distributed between the yoke and leg portions of the core.

BACKGROUND OF THE INVENTION Field of the invention The invention relates in general to magnetic core structures for electrical inductive apparatus, such as electrical transformers, and more particularly to magnetic cores of the stacked type having stepped-lap joints between the leg and yoke portions of the core.

Description of the prior art United States Patent 3,153,215 issued Oct. 13, 1964, which is assigned to the same assignee as the present application, discloses magnetic core structures of the stacked type which utilize stepped-lap joints between the leg and yoke portions of the core. A stepped-lap joint is where the joints between the leg and yoke laminations in each layer of laminations are incrementally offset from similarly located joints in adjacent layers in a predetermined stepped or progressive pattern, with the joints being stepped at least three times in one direction, before the direction is changed or the pattern is repeated. This stepped-lap joint structure was found to substantially improve the performance of the magnetic core, compared to conventional magnetic core structures, an example of which is disclosed in United States Patent 2,300,964, issued Nov. 3, 1942 which is also assigned to the assignee of the present application. It was found that the stepped-lap joint reduced the excitation current requirements of the core, the core had lower losses, and a lower sound level. Further, the hereinbefore mentioned patent 3,153,215 disclosed incremental clipping of the ends of the laminations, which automatically steps the laminations of a stack when the clipped edges are aligned against a plane surface.

On small and medium power transformers, the length of the laminations and the short circuit stresses make it practical to use a relatively small step increment in the stepped-lap pattern, such as /3 of an inch. When applying the stepped-lap pattern to large power transformers, both core form and shell form, it would be desirable to be able to increase the step increment, up to A inch. This would facilitate the stacking of the longer laminations, as the larger the step increment the greater the tolerances in the stacking of the laminations without affecting the losses of the core. Further, a larger step increment is necessary on large power transformers in order to increase the strength of the joints, to withstand the very high short circuit stresses which the magnetic core may be subjected to. However, it was found that for any given stepped pattern, increasing the step increment increases the voids or spaces at the inner corners of the magnetic core produced by the stepped laminations where the leg and yoke por- "ice tions intersect. Since the natural tendency of the magnetic flux is to hug the inner corners of the magnetic core, which ralses the flux density at the inner corners above the average flux density of the core, increasing the void volume at the inner comers of the core is undesirable, as it increases the flux density still further, resulting in increased core losses. This problem was recognized in United States Patent 2,628,273, issued Feb. 10, 1953, which teaches slitting the laminations a predetermined number of times adjacent their end portions, parallel with the longltudinal dimension of the laminations, in order to force the flux away from the inner corners. It would be desirable, however, to reduce the flux density at the inner corners of the magnetic core, without resorting to slitting the laminations.

Further, it would be desirable to be able to incrementally cut the ends of both the leg and yoke laminations, and stack the incrementally cut ends against a plane surface, in order to automatically align the laminations and form the stepped pattern, which eliminates the necrielssity of handling and stacking the laminations individua y.

Further, it would be desirable to be able to incrementally clip either end of a stack of laminations, without regard to whether the incrementally clipped ends of both the leg and yoke portions appear at the same corner of the magnetic core. In the prior art cores, when the clipped portions of the leg and yoke appear at the same corner of the core, it substantially reduces the diagonal joint area of the core.

U.S. Patent 2,628,273, hereinbefore referred to, teaches that the ends of the laminations may be clipped, but the joint progresses around the outer corner of the core. Therefore, the clipped ends of the leg or yoke portions are not aligned, and cannot be used to form the stepped pattern. Therefore, the leg and yoke portions must be individually stacked and aligned, which substantially increases the manufacturing cost of the apparatus.

SUMMARY OF THE INVENTION Briefly, the present invention discloses a new and improved magnetic core structure which, for a given step increment and predetermined stepped pattern, reduces the void volume produced at the inner corners of a stacked type magnetic core utilizing stepped-lap joints. Thus, the step increment can be increased without exceeding the void volume which would be produced in prior art cores using the same stepped pattern with a smaller step increment. Further, the disclosed arrangement for reducing the void volume at the inner corners of the magnetic core, allows both adjoining ends of the yoke and leg portions to be incrementally clipped, without reducing-the diagonal joint area below that of prior art cores in which only one of the stacks of laminations have clipped ends appearing at any one corner of the core. Therefore, the step increment may be increased without reducing the diagonal joint area below that of prior art cores which have both theadjoining ends of the yoke and leg laminations cut, but which have a smaller step increment.

The disclosed magnetic core structure also has the advantage of increasing the throat or entrance area into the leg portions of the magnetic core, for any given step increment or pattern, which reduces the flux density in this area of the core, reducing the losses of the core or, the step increment may be increased without reducing the throat area below that of prior art cores having a smaller step increment.

The reduction in the volume of the voids appearing at the inner corners of the magnetic core is produced, according to the teachings of the invention, by the proper selection of the location of the yoke portions of the core relative to the leg portions, along the diagonal joints between the adjoining ends of the leg and yoke portions. The leg and yoke portions are disposed relative to one another in a manner which balances the inner joint between the leg and yoke portions of the core. Balancing the voids between the yoke and leg portions at the inner corner of the core, also balances the voids at the outer corners of the magnetic core, within the optimum relative position of the yoke and leg portions of the core occurring when the void volume in the yoke portion of the core is substantially equal to the void volume in the leg portion of the core.

BRIEF DESCRIPTION OF THE DRAWING Further advantages of the invention will become more apparent when considered in view of the following detailed description and drawings, in which:

FIGURE 1 is a fragmentary view of a stacked type magnetic core of the prior art, illustrating a stepped-lap joint between the adjoining yoke and leg portions at a corner of the core;

FIG. 1A is a view of the magnetic core shown in FIG. 1, taken in the direction of the arrows I--I, illustrating a typical stepped-lap pattern which may be used;

FIG. 1B is a view of the magnetic core shown in FIG. 1, taken in the direction of the arrows I-I, illustrating another stepped-lap pattern configuration which may be used;

FIG. 2 is a fragmentary view of a stacked type magnetic core, similar to FIG. 1, except illustrating the reduction in diagonal joint area when the adjoining ends of both the yoke and leg laminations are incrementally clipped;

FIG. 2A is a view of the magnetic core shown in FIG. 2, taken in the direction of arrows IIII;

FIG. 3 is a fragmentary view of a stacked type mag netic core constructed according to the teachings of the invention;

FIG. 3A is a view of the magnetic core shown in 3, taken in the direction of the arrows III-III;

FIG. 3B is a perspective view of the magnetic core structure shown in FIG. 3;

FIG. 4 is a plan view of a stacked magnetic core of the single-phase, shell-form type, constructed according to the teachings of the invention;

FIG. 5 is a plan view of a stacked magnetic core illustrating another type of single-phase, shell-form construction, utilizing the teachings of the invention;

FIG. 6 is a plan view of a stacked magnetic core of the three-phase, shell-form type, constructed according to the teachings of the invention;

FIG. 7 is a plan view of a stacked magnetic core illustrating a five legged embodiment of a three-phase, shell-form magnetic core, which utilizes the teachings of the invention;

FIG. 8 is an elevational view of a stacked magnetic core of the single-phase, core-form type, constructed according to the teachings of the invention; and

FIG. 9 is an elevational view of a stacked magnetic core of the three-phase, core-form type, which utilizes the teachings of the invention.

FIG.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, and FIG. 1 in particular, there is shown a fragmentary view of a magnetic core 10 of the stacked type, constructed according to the teachings disclosed in the the hereinbefore mentioned U.S. Patent 3,153,215. FIG. 1 illustrates a corner of the magnetic core 10, formed by intersecting yoke and leg portions 12 and 14 respectively. The yoke portion 12 is formed by stacking or superposing a plurality of metallic laminations 16 formed from a magnetic strip material of the type which has at least one preferred direction of magnetic orientation. The laminations 16 have predetermined longitudinal and width dimensions, with the width dimension being indicated by the letter W. The ends of the laminations are all sheared or cut diagonally with respect to its longitudinal dimension, usually at an angle of 45, in order to form diagonal joints with the leg portions 14, which reduces the losses of the magnetic core. The leg portion 14 is formed by stacking a plurality of metallic laminations 18 formed from the same type of magnetic strip material as the yoke portion 12. The leg laminations 18 have predetermined longitudinal and width dimensions, with the width of the leg laminations 18 being the same as the width of the yoke laminations 16, in this example, also indicated with the letter W. The ends of the leg laminations 18 are all sheared or cut diagonally with respect to their longitudinal dimension, in order to butt up against similar cuts on the yoke laminations 16, which lie in the same layer of laminations.

Magnetic core 10 employs a stepped-lap joint, between the yoke and leg portions 12 and 14, respectively, as well as in the remaining joints of the core. A stepped-lap joint is defined as that type of joint in which similarly located joints in the magnetic core are incrementally offset from one another from layer to layer through the core, with the joints progressing at least three times in a predetermined direction before repeating or changing direction. The stepped pattern may be achieved by forming all of the laminations for a predetermined yoke or leg the same size, and then incrementally clipping a predetermined diagonally cut end, with the number of different incremental cuts, and the incremental dimension being determined by the number of laminations in each basic stepped pattern, and the amount of overlap desired between the steps. For example, in FIG. 1 there are six different clipping dimensions, which when the clipped edges are aligned against a plane surface, along with an unclipped lamination, forms seven steps in one direction. As shown in FIG. 1A, the seven steps may form the basic pattern, repeating itself until the core build dimension is reached; or, as shown in FIG. 1B, the seven steps may form one half of a basic pattern, with the steps moving incrementally in one direction for seven steps, and then moving incrementally in the opposite direction for seven steps. The basic fourteen step pattern may then be repeated until the core build dimension is achieved. The number of steps in a pattern may vary with the application, but it Will include at least three steps in one direction before the pattern is repeated or before the pattern steps in a different direction.

It will be noted that the leg portion 14 which joins the yoke portion 12 does not have incrementally clipped ends, at least on this corner of the magnetic core, thus forming a squared outer corner having no voids in the corner or extensions therefrom. The other end of leg portion 18 may be incrementally clipped to form the stepped pattern or it may be unclipped, with the stepped pattern in the stack of laminations requiring a special fixture having a sloping bottom and an angled stop, in order to align the laminations into a stepped pattern. It should also be noted that the stepped type pattern produces voids 20 at the inner corner of the magnetic core 10, with the voids 20 appearing in the leg portion 18 as progressively larger triangles, as the pattern steps in a. predetermined direction. The stepped-lap type pattern, thus distorts the magnetic flux at the inner corner of the magnetic core, increasing the flux density at an area which already has a higher flux density than the average flux density of the core, due to the natural tendency of the magnetic flux to hug the inner corners of the core. While the voids 20 add to the losses of the magnetic core, the stepped-lap joints are so superior in performance to the butt-lap type joint, that the overall performance of the magnetic core having stepped-lap joints, at least using a stepped increment of /8 of an inch, is better than the performance of the magnetic cores which use butt-lap joints.

Further, the stepped-lap pattern reduces the diagonal joint area so that shown by the dimension Y in FIG. 1,

and it reduces the throat area into the legs of the magnetic core, to that shown by the dimension L.

When the magnetic core is for small and medium power transformers, up to approximately 5000' kva., the length of the laminations and the short circuit stresses are such that it is practical to utilize a step increment of /8 inch from layer to layer. Thus, the voids produced at the inner corners of the core, the reduction in the diagonal joint area, and the reduction in the throat area into the leg portions, do not present serious problems, due to the hereinbefore mentioned superiority of the stepped-lap joint over the butt-lap joint electrically, mechanically, and sound-wise, more than offsetting the disadvantages due to the inner corner voids, reduction in diagonal joint area, and reduction in throat area into the legs. It would be desirable, however, for a given step increment, to reduce the volume of the inner corner voids, and to increase the diagonal joint and throat areas, in order to improve its performance still further, and also to be able to increase the step increment without reducing the performance of the core below prior art cores having smaller step increments.

As shown in FIG. 1, the ends of the leg portion 14 which adjoin the yoke portion 12 are not incrementally clipped. FIG. 2 illustrates a magnetic core 30, constructed in a manner similar to the magnetic core 10 shown in FIG. 1, having yoke and leg portions 32 and 40, respectively. Yoke portion 32 is formed of a plurality of stacked laminations 34, each having diagonally cut ends, and incremental clips on at least one of the diagonally cut ends. In this instance, the incremental clips have been applied to all of the laminations 34, in order to eliminate having some laminations with sharp points which are easily bent during handling. Leg portion 40 is formed of a plurality of stacked laminations 42, having diagonally cut ends, and, unlike the leg portion 14 shown in FIG. 1, leg portion 40 has its ends incrementally clipped. The voids or spaces 44, which are formed at the inner corner of the core, have the same volume as the voids 20 in the magnetic core 10, the minimum throat dimension L is the same as in the magnetic core 10, but the minimum diagonal joint dimension has been reduced due to the incrementally clipped ends on the leg portion 40. Instead of the minimum diagonal joint dimension being Y as shown in FIG. 1, it is reduced to a value of YAY. It would be desirable, for a given step increment, to be able to increase the diagonal joint area, so that the performance of the core may be improved, or to allow the step increment to be increased without substantially changing its performance from prior art cores having smaller incremental steps.

FIG. 2A is a view of the magnetic core 30 shown in FIG. 2, taken in the direction of the arrows II-II, illustrating the voids or spaces. 33 produced at the outer corner of the core by clipping both the yoke and leg portions of the core at the same corner.

Since the stepped-lap joint is superior in performance to the butt-lap joint, it would be desirable to be able to apply it to the cores of large power transformers, approximately 5000 kva. and above. The longer laminations and the higher short circuit stresses, however, make it impractical to utilize a step increment of /8 of an inch. The problem of handling and stacking these long laminations, and aligning them against a plane surface would be alleviated by using a larger step increment, as the larger the step increment the greater the mechanical tolerance, without deleteriously affecting the performance of the core. Further, in these larger power transformers, a larger step increment is necessary in order to increase the strength of the joint to withstand the tremendous short circuit stresses which the core and its windings may be subjected to. Thus, it would be desirable to be able to increase the step increment up to one-quarter inch. In the prior art, increasing the incremental step dimension increases the volume of the voids at the inner corners of the core, it reduces the diagonal joint area, and it reduces the throat area into the legs, all of which contribute to flux crowding and higher flux densities which increase the losses of the core. These problems are not solved by merely reducing the maximum number of steps in one direction when increasing the step increment, as the core losses are increased as the number of steps in one direction are decreased, and the joint strength is increased as the number of steps in one direction are increased. Thus, it would be desirable to be able to increase the step increment without reducing the number of steps in one direction by the same ratio, without significantly increasing the void volume at the inner corner of the core, and without significantly reducing the diagonal joint and throat areas into the legs. Further, it would be desirable to be able to have both the adjoining ends of the leg and yoke laminations clipped without reducing the diagonal joint area of the core.

FIG. 3 is a fragmentary view of a stacked type magnetic core 50, constructed according to the teachings of the invention, which for a given step increment reduces the void volume at the inner corners of the core, and increases the diagonal joint and throat areas. Thus, the disclosed construction allows the step increment to be increased and allows the ends of adjoining yoke and leg laminations to be clipped, without substantially increasing the void volume at the inner corners of the core, and without substantially reducing the diagonal joint area and the throat area into the leg portions of the core.

More specifically FIG. 3 illustrates a magnetic core 50 having yoke and leg portions 52 and 54, respectively. The yoke and leg portions 52 and 54 may be constructed similarly to the yoke and leg portions 32 and 40 of the magnetic core 30 shown in FIG. 2, with the yoke portion 52 including a plurality of stacked laminations 56, and the leg portion 54 including a plurality of stacked laminations 58. The adjoining ends of the laminations which make-up the yoke and leg portions 52 and 54 are cut diagonally with respect to their longitudinal dimensions, to form diagonal joints 60 in each layer of the core. The ends of the yoke and leg laminations are incrementally clipped at 62 and 64, respectively, to form stepped-lap joints by aligning the clipped ends of the laminations. The clipping increments utilized determine the joint overlap between adjacent layers. For ease in comparing the magnetic core structure shown in FIG. 3 with the magnetic core structures shown in FIGS. 1 and 2, the lamination width W, the number of laminations stepped in a predetermined direction, and the stepped increment are the same in all three of these figures.

Instead of aligning the yoke and leg portions of the magnetic core to form a squared outer corner, such as shown in FIG. 1, or to form an outer corner which would be square if the clipped ends were to be restored, such as shown in FIG. 2, the position of the yoke portion 52 relative to to leg portion 54 is changed, along the direction of the diagonal joint. The new relative positions of the yoke and leg portions, shown in FIG. 3, may be best explained by first considering the structure shown in FIG. 2, which has leg and yoke portions which are identical to the yoke and leg portions shown in FIG. 3, except for their relative positions. If the yoke portion 32 of the magnetic core 30 shown in FIG. 2 is moved in the direction of the diagonal joint, from the outer to the inner corner, the voids 44 in the leg portion 40 will start to be filled, and voids will begin to appear in the yoke portion. The yoke portion is moved until the voids appearing in the yoke and leg portion are substantially equal in volume, which is the optimum location. Thus, in FIG. 3 the voids 66 in the leg portion 54 are equal in volume to the voids 68 in the yoke portion 52. The total void volume at the inner corner of the magnetic core 50 shown in FIG. 3, is less than the total void volume appearing at the inner corner of the magnetic core 30 shown in FIG. 2. This may be easily shown by comparing the area of the maximum void space at the inner corners of the magnetic cores 30 and 50. If the horizontal dimension of the largest void in FIG. 2 is given the dimension b, and the vertical dimension is given the dimension h, the area of the largest void will be b h/ 2. In FIG. 3, the relative positions of the yoke and leg portions are changed such that the b and h dimensions are bisected. Therefore, the horizontal dimension of the largest void, in both the yoke and leg portions is b 2, and the vertical dimension is h/ 2, which produces an area of the bh/ 8 for the yoke portion, and bh/ 8 for the leg portion, or a total area of bh/ 4, which is one half of the area of the largest void in the magnetic core structure shown in FIG. 2. Thus, using the same stepped pattern arrangement, the teachings of the invention provide a reduction in the void volume at the inner corner of the core by 50%, thus reducing the. distortion of the magnetic flux at the inner corner, reducing flux crowding, and reducing the flux density at the inner corners of the core, which reduces the losses of the magnetic core. Or, the teachings of the invention may be used to increase the step increment, which will then increase the volume of the voids in the leg and yoke portions of the core. A larger step increment may be chosen which will increase the largest void area to bh/2, without exceeding the losses of a prior art core having a smaller step increment.

Further, it will be observed from FIG. 3, that the minimum diagonal joint dimension is equal to Y, the same as the minimum diagonal joint dimension of the prior art core shown in FIG. 1. Thus, the clipping of the leg portion 54 has been accomplished by the magnetic core structure shown in FIG. 3, without the loss in the diagonal joint area, as shown in FIG. 2. Therefore, using the same stepped pattern and stepped increment as in prior art core construction, FIG. 3 teaches how to utilize clipping on the adjacent ends of the yoke and leg laminations, without the loss of diagonal joint area as shown in FIG. .2. Or, the teachings of FIG. 3 will allow the step increment to be increased until reduction in joint dimension is AY, without reducing the joint dimension below that of FIG. 2, which utilizes a smaller step increment.

In both the magnetic core structure shown in FIGS. 1 and 2, the minimum dimension leading into the leg portion, which will be called the throat dimension, is equal to L. In the magnetic core structure shown in FIG. 3, the throat dimension is increased to L-f-AL. Thus, if the same stepped pattern and increment is used, the teachings of the invention illustrated in FIG. 3 allow the flux density entering the legs to be reduced, thus reducing the losses of the magnetic core. Or, using the teachings of the invention, the stepped increment can be increased until the throat dimension is reduced to the dimension L, which will provide the same flux density in this area of the core as the prior art cores which utilize a smaller stepped increment.

FIG. 3A is a view of the outer corner of the magnetic core 50 shown in FIG. 3, taken in the direction of the arrows IIIIII. FIG. 3A illustrates how voids 70 are formed in the yoke portion 52 and voids 72 are formed in the leg portion 54. The voids are distributed substantially equally between the yoke and leg portions, and like the inner corner of the core, the maximum void volume at the outer corner of the core has been reduced, compared to the structure shown in FIG. 2.

If the incrementally clipped ends of the yoke and leg portions 52 and 54 of the magnetic core 50 shown in FIG. 3 were to be restored, it would be noted that the stepped joint appears on both sides of the outer corner. In the prior art core of FIG. 1, the stepped joint appears on only one side of the corner, which was essential in order to be able to use the incrementally clipped ends on the yoke to form the stepped pattern. In FIG. 3, the stepped pattern is formed, on both the leg and yoke portions, by placing the incrementally clipped ends on the yoke laminations against a plane surface, and by placing the incrementally clipped ends on the leg laminations against a plane surface. The complete yoke and leg portions may then be independently stacked and then placed together to form the diagonal step lap joint, without laboriously stacking the magnetic core one layer at a time, which would be necessary in the prior art magnetic cores in which the joint steps or progresses around the outer corners of the core.

FIG. 3B is an enlarged perspective view taken of the outer corner of the magnetic core 50, which clearly illustrates the formation of the voids 70 and 72 and in the yoke and leg portions, respectively, formed by the incrementally clipped edges 62 and 64 of the laminations 56 and 58, respectively.

While FIG. 3 illustrates a magnetic core of the type in which the yoke and leg laminations have the same width, which is the conventional construction for shellform magnetic core structures, and certain core-form, magnetic core structures, it will be understood that the teachings of the invention apply equally to the situation where the yoke is widened, which structure is sometimes utilized in core-form transformers.

The teachings of the invention may be applied to single and polyphase transformers having magnetic cores of the core-form type or magnetic cores of the shell-form type, possessing advantages over the prior art cores on small and medium power transformers, even when the same step increment is used, due to the reduction in the void volume at the inner corners of the core, the greater diagonal joint area, and the greater throat area into the leg portions of the core, and allowing the advantages of the steppedlap construction to be extended to large power transformers, by permitting the stepped increment to be increased without significantly increasing the losses beyond those associated with prior art cores used in the smaller transformer ratings.

FIGS. 4 through 9 illustrate several embodiments of the invention, but it is to be understood that these embodiments are given as examples, and they are not meant to limit the invention to the specific structures illustrated.

FIG. 4 is a plan view of a transformer of the singlephase, shell-form construction, having a magnetic core 82 and electrical windings 84 disposed in inductive relation therewith. Magnetic core 82 has upper and lower yoke portions 86 and 88, respectively, which join leg portions 90, 92 and 94. The upper yoke portion 86, in this instance, is formed of two separate laminations 96 and 98 in each layer of the core, with the width of the laminations being indicated by the letter W. The lower yoke portion 88 also has two laminations 100 and 102 per layer, similar to the upper yoke portion, with the width of the lower yoke portion also being equal to W. The outer leg portions and 94 of the magnetic core 82 are formed from stacked laminations 104 and 108, respectively, with each having a width dimension W. The inner leg portion 92 of the magnetic core 82, which in this type of magnetic core construction forms the winding leg, is formed of a plurality of stacked laminations 106, which have a width equal to 2W. The flux in the inner leg of the magnetic core is twice that in the outer legs, and in order to maintain the same flux density throughout the magnetic core, the cross-sectional area of the inner leg should be at least twice the cross sectional area of each of the outer legs.

Each of the joints in the magnetic core 82 are balanced, with the void volumes at the inner corners of the core appearing equally in the yoke and leg portions of the core. The joints at opposite corners of the core are similar, with the outer corner 81 of the magnetic core being similar to the corner on the magnetic core 50 shown in FIG. 3. Similarly, the outer corner 83- formed between laminations and 104 is similar to the corner shown in FIG. 3. Since the number of steps in each of the stepped lap joints will depend upon the specific application, only three joints are shown in the figure for purposes of simplicity, with any additional joints appears,477,05s i ing between the two outer joints shown in the drawing. Since the incremental clips on the laminations 108 and 93, appearing at the outer corner 81 of the magnetic core, will automatically align these laminations into the proper stepped lap pattern, corner 85 of the core may have a somewhat ditferent appearance than the corners 81 and 83. For example, laminations 108 may have their diagonally cut ends extending beyond the sides of the core, as illustrated in FIG. 4, or they may be clipped oil, if desired. In this embodiment of the invention the portions of the yoke for-med by laminations 96 and 102 will not be able to be formed by incrementally clipping the ends. In this instance, a special fixture having a sloping bottom and an angled stop may be used to automatically align the laminations into the predetermined stepped pattern.

The inner leg portion 92 of magnetic core 82 also forms a stepped lap pattern with the upper and lower yoke portions, with a balanced joint being provided which equally divides the voids between the yoke and leg portions by maintaining a constant angle on the spear point ends and incrementally moving the end of the spear point perpendicular to the longitudinal dimension of the leg laminations.

FIG. illustrates another form of a single-phase, shellform transformer which may utilize the teachings of the invention. More specifically, FIG. 5 illustrates a transformer 110 having a magnetic core structure 112 and a winding assembly 114 disposed in inductive relation therewith. In this particular embodiment, the magnetic core structure 112 is formed of two similar magnetic core sections 116 and 118. The winding leg of the magnetic core 112 is formed by disposing what would normally be thought of as the yoke portions of the two sections in side-by-side relation, with the winding assembly 114 being disposed about the resulting leg. Since the two sections of the magnetic core 112 are similar in construction, only section 116 will be described in detail. Magnetic core section 116 has what will be called upper and lower yoke portions 124 and 126, which connect leg portions 116 and 120. When actually assembled into the final structure, the upper yoke portion 124 forms one of the outer leg portions of the core, lower yoke portion 126 forms one-half of the winding leg, and the leg portions 116 and 120 form part of the yoke structure. Leg portion 122 is formed of a plurality of laminations 130 having a width W, with both ends of the laminations being diagonally cut and the lower end of the laminations being incrementally clipped at corner 113. The opposite end may have a constant clip on the laminations, in order to insure that the laminations will not extend beyond the side of the core. Leg portion 120, in like manner, is for-med of a plurality of stacked laminations 128 having a width W, with the ends of the laminations being diagonally cut, and one end incrementally clipped at corner 111, and the other end having a constant clip at corner 117. In this instance, the constant clip on the laminations have a practical purpose, as it is essential that they do not extend past the sides of the core section, in order to prevent them from interferring with the adjacent core section 118. The upper yoke portion 124 is formed of a plurality of stacked laminations 13-2 having a width dimension W, diagonally cut ends, and incrementally cut-ends which appear at corner 111. The lower yoke portion 126 has a plurality of stacked laminations 134 having a width dimension W, diagonally cut ends, and incrementally cut ends which appear at outer corner 113. All of the corner joints are balanced, as disclosed by the teachings of the invention shown in FIG. 3, wherein half of the void volume appears in both the leg and yoke laminations at the inner corners of the core. Corners 111 and 113 will appear as shown in FIG. 3, and outer corners 115 and 117 will be similar to one another having voids produced by the constant clip on the ends of the leg laminations.

FIG. 6 is a plan view of a three-phase transformer 1 40 'of the shell-form type, which utilizes the teachings of the invention. Transformer includes a magnetic core structure 142 having two similar magnetic core sections'152 and 154 disposed in side-by-side relation, with their adjoining portions forming winding legs for receiving the phase winding assemblies 146, 148 and 150. Since the magnetic core structures 152 and 154 are similar in construction, only section 152 will be described in detail. Magnetic core section 152 includes an upper yoke portion 153 and a lower yoke portion 155, which joins outer leg portions 157 and 163 and intermediate leg portions 159 and 161. The upper and lower yoke portions are shown divided, having more than one lamination per layer, for the purpose of making the laminations shorter and easier to handle. It will be understood, however, thatthese yoke portions may be formed of a single lamination per layer or more than two laminations per layer. For example, as shown in FIG. 6, the upper yoke portion 153 is constructed of a plurality of stacked la-minations 156 and 158, and the lower yoke portion is constructed of a plurality of stacked laminations 160 and 162. All of the laminations have diagonally cut ends, with the ends of laminations 158 incrementally clipped at corner 165, and the ends of laminations 160 being in crementally clipped at corner 167. The outer leg portions 157 and 163 are formed of a plurality of stacked laminations 164 and 170, respectively, and the intermediate leg portions 159 and 161 are for-med of a plurality of stacked laminations 166 and 168, respectively. All of the laminations have the same width dimension W. All of the inner corners of the magnetic core section have a balanced joint, with the void volume being equally divided between the adjoining leg and yoke portions, according to the teachings of the invention. Outer corners and 167 are formed in the manner illustrated in FIG. 3 and outer corners 169 and 171 will have voids therein formed by the constant clips on the ends of the leg laminations 164 and The balanced stepped-lap joint between the intermediate leg portions 159 and 161 and the yoke portions is formed in the manner hereinbefore described relative to FIG. 4. When the two portions or sections of the magnetic core 142 are placed adjacent one another, the adjacent yoke portions form the winding leg, and the outer yoke portions form the outer legs of the core, with the leg portions of the core acting as yokes.

Tests performed on a magnetic core constructed as shown in FIG. 6, at an average shell-form induction of sixteen kilogauss, showed an improvement of 53.8% in excitation current an improvement of 6.7% in watts loss, and a 9.9 db sound level advantage over a similarly rated conventional core constructed with butt-lap joints.

Instead of using two separate magnetic core sections to form the magnetic core for a three-phase, shell-form transformer, the teachings of the invention make it possible to lower the noise level of a five-legged magnetic core, and also lower its losses to a point where it is practical to form a three-phase, shell-form transformer as shown in FIG. 7. FIG. 7 illustrates a transformer of the three-phase, shell-form type having a single magnetic core structure 182 and phase windings 184, 186 and 188 disposed in inductive relation therewith. Magnetic core structure 182 has five leg portions 194, 196, 198, 200 and 202 with the outer leg portions 194 and 202 having a width dimension W, and the intermediate leg portions 196, 198 and 200, having a width dimension equal to 2W. The outer leg portions 194 and 202 are formed of a plurality of stacked laminations 220 and 228, respectively, which have their ends cut diagonally, and the lower end of the laminations 220 incrementally clipped at outer corner 221, and laminations 228 having their upper end incrementally clipped at outer corner 223. The remaining ends of laminations 220 and 228 may be left unclipped, as any extension beyond the sides of the core will not be objectionable. Yoke portion has a plurality of laminations per layer 204, 206, 208 and 210 with the stacks of laminations 210 and 204 having their ends incrementally clipped at corners 223 and 225. The remaining laminations of yoke portion 190 will have to be stacked in a special fixture, as hereinbefore described. Yoke portion 192 is similar to yoke portion 190, having four laminations per layer 212, 214, 216 and 218, with the laminations 212 and 218 being incrementally clipped at corners 221 and 227. The remaining laminations will have to be aligned in a special fixture. All of the inner corners of the magnetic core have a balanced joint design, with half of the void volume appearing in the yoke portion and half in the leg portion of the core. The balanced joint between the intermediate leg portions and the yoke portions is formed as hereinbefore described relative to FIG. 4.

The teachings of the invention may also be applied to transformers of the core-form type. FIG. 8 is an elevational view of a single-phase transformer 230 of the coreform type having a magnetic core structure 232 and windings 234 and 236 disposed in inductive relation therewith. The single phase windings are shown split and disposed in inductive relation with both legs, but the windings may be disposed on one leg, if desired. Magnetic core 232 includes leg portions 242 and 244 joined by upper and lower yoke portions 232 and 240, respectively. The leg portions 242 and 244 are formed of a plurality of stacked laminations 250 and 252, respectively, having their ends diagonally cut and one of their ends incrementally cut to form corners 255 and 257. Yoke portions 238 and 240' are formed of a plurality of stacked laminations 246 and 248, respectively, having their ends diagonally cut and one of their ends incrementally cut to form corners 255 and 257. The remain ing ends of yoke laminations 246 and 248 may be cut to a constant dimension, if desired, to prevent the laminations from extending past the sides of the magnetic core. All of the inner corners of the magnetic core have a balanced joint, distributed equally between the yoke and leg portions, and the outer corners 2'55 and 257 have an appearance as shown in FIG. 3.

The teachings of the invention may also be extended to three-phase, core-form transformers, as shown in FIG. 9. FIG. 9 illustrates a three-phase transformer 260 of the core-form type having a magnetic core structure 262 and phase winding assemblies 264, 266 and 268 disposed in inductive relation therewith. Magnetic core structure 262 includes upper and lower yoke portions 270 and 272, respectively, which join leg portions 274, 276 and 278. The upper yoke portion 270 is shown divided, having laminations 280 and 282 in each layer, but it may have a single piece per layer if desired. The ends of laminations 280 and 282 are diagonally cut, with one end of the laminations 280 being incrementally cut at corner 281. Laminations 282 may be stacked in a special fixture, and they may include laminations which extend beyond the edge of the core at corner 287 if desired, or they may be clipped if necessary. The lower yoke portion 272 includes a plurality of stacked laminations 284, which have their ends diagonally cut, and one end incrementally cut to form corner 283. The leg portions 274, 276 and 278 are formed of a plurality of stacked laminations 286, 288 and 290, respectively, with the outer laminations 286 and 290 having diagonally cut ends and one end incrementally cut to form corners 281 and 283. The inner leg portion 276 has its laminations cut into a spear point, as hereinbefore described relative to FIG. 4. All of the inner corners of the magnetic core have balanced joints, appearing in both the leg and yoke portions, and the outer corners 281 and 283 have an appearance as shown in FIG. 3.

In summary, the teachings of the invention may be applied to any type of magnetic core structure, single or poly-phase, core-form or shell-form, which improves the performance of the core compared to stepped-lap cores of the prior art, and which allows the advantages of the stepped-lap construction to be extended into higher power transformer ratings without exceeding the percentage change of losses and sound levels of smaller rated prior art transformers.

Since numerous changes may be made in the abovedescribed apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

We claim as our invention:

1. A magnetic core comprising a plurality of superposed layers of laminations; said laminations having predetermined width and longitudinal dimensions; each of said layers of laminations including at least two yoke laminations and at least two leg laminations, forming a structure having at least first and second leg portions connected by first and second yoke portions, a plurality of outer corners and associated inner corners; the yoke and leg laminations of each layer having at least one of their ends cut diagonally with respect to the longitudinal dimension, providing a closed magnetic circuit having diagonal joints between adjoining ends of the leg and yoke laminations; the diagonal joints in each layer of laminations being incrementally ofiset from similar joints in adjacent layers to provide a predetermined stepped-lap pattern having at least three steps in one direction; the diagonally cut adjoining ends of the laminations of predetermined yoke and leg portions being incrementally clipped at right angles to their longitudinal dimensions, with all of the clipped ends in each portion being aligned to provide the predetermined stepped-lap pattern; the relative location of adjoining leg and yoke laminations in the direction of their diagonal joint, being selected to divide the voids formed by the stepped laminations at the intersection of the leg and yoke laminations between certain associated inner and outer corners of the magnetic core structure, and to divide the voids in the inner corners of the magnetic core structure between the yoke and leg portions of the core.

2. The magnetic core of claim 1 wherein the voids are substantially equally divided between the yoke and leg laminations at the inner corners of the core structure.

3. The magnetic core of claim 1 wherein the incrementally clipped ends of the leg and yoke portions are disposed at the same outer corners of the magnetic core.

4. The magnetic core of claim 3 wherein the widths of the leg and yoke laminations are substantially the same, and the relative location of the yoke and leg portions is selected to divide the voids equally between at least the outer corners of the magnetic core having the incrementally clipped ends and their associated inner corners.

5. The magnetic core of claim 1 wherein each layer of laminations includes a third leg lamination which provides an intermediate leg portion in the magnetic core structure, with the intermediate leg portion joining the yoke portion with a stepped-lap joint having at least three steps in one direction, and with the voids formed at the intersection of the intermediate leg portion and the yoke portions being substantially equally divided between the intermediate leg portions and the yoke portions.

6. The magnetic core of claim 5 wherein the intermediate leg portion is substantially the same width as said first and second leg portions.

7. The magnetic core of claim 5 wherein the intermediate leg portion is substantially twice the width of said first and second leg portions.

8. The magnetic core of claim 1 wherein each layer of laminations includes third and fourth leg laminations which provide two intermediate leg portions in the magnetic core structure, with the intermediate leg portions with stepped-lep joints having at least three steps in one direction, and with the voids formed between the intermediate leg portions and the yoke portions being substantially equally divided therebetween.

9. The magnetic core of claim 8 wherein it forms one section of a three-phase, shell-form magnetic core structure having two similar sections, with the intermediate leg portion being substantially the same width as the first and second leg portions.

10. The magnetic core of claim 1 wherein each layer of laminations includes third, fourth and fifth leg laminations which provide three intermediate leg portions in the magnetic core structure, with the intermediate leg portions joining the yoke portions with stepped-lap joints having at least three steps in one direction, and wherein the voids formed between the intermediate leg portions and the yoke portions are divided substantially equally therebetween.

11. The magnetic core of claim 10 wherein the intermediate leg portions are at least twice as wide as the first and second leg portions.

References Cited UNITED STATES PATENTS 577,480 2/1897 Gutmann 336-214 2,628,273 2/1953 'Somerville 336--217 5 3,082,391 3/1963 Chiki 336210 3,153,215 10/1964 Burkhardt et a1. 336-217 3,210,709 10/1965 Ellis et al. 336 217 FOREIGN PATENTS 1,256,000 2/1961 France. 1,354,694 1/1964 France.

LEWIS H. MYERS, Primary Examiner 15 T. J. KOZMA, Assistant Examiner US. Cl. X.R. 336215, 217 

